Postfix issue: Relay access denied

I've browsed this forum endlessly and read countless posts and I cannot figure how to fix the problem. My postfix is setup step by step based on the ISPConfig setup for Hardy Ubuntu.
The issue is that when I try to send emails to the users in postfix i get a Delivery Status Notification (Failure). It states:

PERM_FAILURE: Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 554 554 5.7.1 <[email protected]>: Recipient address rejected: Relay access denied (state 14).

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When I send a email from the postfix module on the server, I am able to successfully send out emails. Except the email that gets sent out is from the user [email protected]. And when I try to reply to that email I do not immediately get a Delivery Status Notification.....
Hours later I received this:

Please create an A record for server.mydomain.com.
Also make sure that you use "Server requires authentication" in your email client and that your server is not blacklisted: http://mxtoolbox.com/blacklists.aspx